Subnetting Cheat Sheet: Top Tips for CCNA and Network+ Exams

Subnetting is a core skill for networking certifications like CCNA and CompTIA Network+, but it can feel overwhelming with all the math and terminology. Whether you’re dividing networks with FLSM or VLSM, summarizing routes with supernetting, or tackling IPv6, this cheat sheet provides key formulas, tips, and practice problems to boost your confidence. Designed for beginners and exam candidates, this guide summarizes concepts from our blog series and shows how our All-in-One Subnet Calculator can simplify your prep. Let’s dive in!

Key Subnetting Concepts

Subnetting divides a large network into smaller sub-networks for organization, security, and efficiency. Here’s a quick overview of the main techniques we’ve covered:

• FLSM (Fixed Length Subnet Masking): Creates equal-sized subnets. Simple but can waste IPs. Learn more in our FLSM guide.
• VLSM (Variable Length Subnet Masking): Creates subnets of different sizes to match host needs, saving IPs. See our VLSM guide.
• Supernetting: Combines subnets into a larger network to simplify routing. Check our Supernetting guide.
• IPv6 Subnetting: Divides massive 128-bit address spaces, typically into /64 subnets. Explore our IPv6 Subnetting guide.

Our Subnet Calculator supports all these techniques for quick calculations.

Essential Subnetting Formulas

Memorize these formulas for IPv4 subnetting success:

• Number of Subnets: 2^n, where n = bits borrowed from the host portion.
• Hosts per Subnet: 2^h - 2, where h = host bits (subtract 2 for network and broadcast addresses).
• Subnet Increment: 256 / 2^n (for the last octet in Class C, adjust for other classes).
• CIDR to Subnet Mask: /24 = 255.255.255.0, /25 = 255.255.255.128, etc. Use our IPv4 Chart for reference.

For IPv6, focus on prefix lengths (e.g., /48 to /64) and hexadecimal subnet IDs. See our IPv6 Chart for details.

Quick Reference Table

Use this table for IPv4 subnetting calculations:

CIDR	Subnet Mask	Usable Hosts	Increment	Subnets (Class C)
/24	255.255.255.0	254	256	1
/25	255.255.255.128	126	128	2
/26	255.255.255.192	62	64	4
/27	255.255.255.224	30	32	8
/28	255.255.255.240	14	16	16

For more CIDR values, check our IPv4 Chart and IPv6 Chart.

Top 5 Subnetting Tips for Exams

1. Memorize Powers of 2: Know 2^1 to 2^10 (2, 4, 8, 16, 32, 64, 128, 256, 512, 1024) for quick subnet and host calculations.
2. Practice Binary Math: Convert IP addresses and masks to binary for FLSM, VLSM, and supernetting. Use our Subnet Calculator to verify.
3. Understand CIDR: Learn how CIDR notation (e.g., /26) translates to subnet masks and host counts. Reference our IPv4 Chart.
4. Master VLSM for Efficiency: Practice allocating subnets by host needs (largest to smallest) to avoid overlaps. Try our VLSM Calculator.
5. Learn IPv6 Basics: Focus on /64 subnets and hexadecimal subnet IDs for IPv6 questions. Use our IPv6 Calculator for practice.

Practice Problem: IPv4 Subnetting

Task: Subnet 172.16.0.0/16 into 8 equal subnets using FLSM.

Solution:

1. Calculate Subnets: Borrow 3 bits (2^3 = 8 subnets). New mask: /19 (16 + 3).
2. Hosts per Subnet: 13 host bits (2^13 - 2 = 8,190 usable hosts).
3. Increment: 2,048 (32,768 ÷ 16, Class B adjustment).
4. Subnet Ranges:
   • Subnet 1: 172.16.0.0–172.16.7.255 (/19)
   • Subnet 2: 172.16.8.0–172.16.15.255 (/19)
   • Subnet 3: 172.16.16.0–172.16.23.255 (/19)
   • ... (up to Subnet 8).
5. Verify: Use our FLSM Calculator to input 172.16.0.0/16 and select 8 subnets.

Practice Problem: IPv6 Subnetting

Task: Subnet 2001:0db8:5678::/48 into 4 /64 subnets.

Solution:

1. Calculate Subnets: Use 16 bits for subnet ID (48 to 64). 2^16 = 65,536 possible /64 subnets.
2. Assign Subnets:
   • Subnet 1: 2001:0db8:5678:0000::/64
   • Subnet 2: 2001:0db8:5678:0001::/64
   • Subnet 3: 2001:0db8:5678:0002::/64
   • Subnet 4: 2001:0db8:5678:0003::/64
3. Verify: Use our IPv6 Calculator to input 2001:0db8:5678::/48 and generate /64 subnets.

Common Exam Mistakes to Avoid

• Ignoring Reserved IPs: In IPv4, exclude network and broadcast addresses from usable hosts.
• Misjudging Subnet Size: For VLSM, allocate largest subnets first to avoid overlaps. Use our VLSM Calculator.
• IPv6 Prefix Errors: Stick to /64 for LANs in IPv6 to support SLAAC. Check our IPv6 Chart.
• Supernetting Non-Contiguous Subnets: Ensure subnets are adjacent (e.g., 192.168.0.0/24 and 192.168.1.0/24). Use our Supernetting tool.

Why Use Our Calculators?

Manual subnetting is time-consuming and error-prone, especially under exam pressure. Our tools make it easy:

• FLSM Calculator: For equal-sized subnets.
• VLSM Calculator: For variable-sized subnets.
• Supernetting: For route summarization.
• IPv6 Calculator: For modern networks.
• All-in-One Subnet Calculator: Combines all features.

Use our IPv4 Chart and IPv6 Chart for quick reference during practice.

Next Steps: Ace Your Exam

Subnetting is a critical exam topic, but practice makes perfect. Here’s how to prepare:

1. Practice Problems: Try subnetting 192.168.1.0/24 with VLSM for 50, 20, and 10 hosts using our VLSM Calculator.
2. Simulate Exams: Time yourself solving subnetting questions, verifying with our Subnet Calculator.
3. Review Charts: Memorize key CIDR and prefix values using our IPv4 Chart and IPv6 Chart.